A High speed Ethernet transmitter typically includes an optical transmitter. A directly modulated laser can be used as the optical signal source of the transmitter to provide high optical output power at relatively low cost. To reduce power consumption and to improve high speed electrical signal quality, a laser driver can be packaged inside the transmitter housing, in close proximity to the laser. The laser driver in such a transmitter is generally mounted to a substrate coupled to a microwave waveguide to direct high frequency signals from outside the transmitter housing to the laser driver. The laser driver can heat up during use, and some of this heat can be transferred to the laser due to its close proximity with the laser driver. Such heat transfer results in decreased laser performance.